Virtual machine image server

ABSTRACT

Virtual machine image files are stored in a virtual machine storage by a virtual machine image file server. Users at virtual machine clients connect to the virtual machine image file server and are presented with a list of the available virtual machine image files in a virtual machine image store. Users of the virtual machine clients are able to select the desired virtual machine image, and begin executing in the selected virtual machine environment as soon as the selected virtual machine image file begins downloading to the client computer. When a virtual machine image file is selected for storage it is divided into several segments. Each segment is compared with segments of previously stored virtual machine image files already in storage. For example, only non-duplicate segments are stored.

CROSS REFERENCE TO RELATED APPLICATIONS

This invention is related to the following pending U.S. PatentApplications: U.S. patent application Ser. No. 10/825,735, filed on Apr.15, 2004, now U.S. Pat. No. 7,555,531, issued on Jun. 30, 2009,“EFFICIENT ALGORITHM AND PROTOCOL FOR REMOTE DIFFERENTIAL COMPRESSION”;U.S. patent application Ser. No. 10/844,893, filed on May 13, 2004,“EFFICIENT CHUNKING ALGORITHM”; U.S. patent application Ser. No.10/844,906, filed on May 13, 2004, “EFFICIENT ALGORITHM AND PROTOCOL FORREMOTE DIFFERENTIAL COMPRESSION ON A REMOTE DEVICE”; U.S. patentapplication Ser. No. 10/844,907, filed on May 13, 2004, “EFFICIENTALGORITHM AND PROTOCOL FOR REMOTE DIFFERENTIAL COMPRESSION ON A LOCALDEVICE”; and U.S. patent application Ser. No. 10/948,980, filed on Sep.24, 2004, now U.S. Pat. No. 7,613,787, issued on Nov. 3, 2009,“EFFICIENT ALGORITHM AND PROTOCOL FOR FINDING CANDIDATE OBJECTS FORREMOTE DIFFERENTIAL COMPRESSION”. The invention is further related tothe U.S. patent application Ser. No. 11/031,158, filed on Jan. 7, 2005,now U.S. Pat. No. 7,849,462, issued on Dec. 7, 2010, “IMAGE SERVER”. Thecontents of the above applications are hereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to the field of virtual machines. Morespecifically this invention relates to system and methods for storingand serving virtual machine image files.

BACKGROUND OF THE INVENTION

Virtual machines enable a host computer to run multiple applicationenvironments or operating systems on the same computer simultaneously.The host computer allots a certain amount of the host's resources toeach of the virtual machines. Each virtual machine is then able to usethe allotted resources to execute applications, including operatingsystems. The virtual machine virtualizes the underlying hardware of thehost computer or emulates hardware devices, making the use of thevirtual machine transparent to the operating system or the user of theapplication. Typical virtual machines make use of virtual machine imagefiles to store the desired application environment or operating system.One common type of virtual machine image file is the virtual hard drive(“VHD”). To the host system, a VHD is simply a large file that can becopied and backed up and to which standard file system permissions canbe applied. To the virtual machine, the VHD file appears to be a fullhard drive, and typically contains an operating system and a set ofapplications.

For modern operating systems, virtual machine image files can typicallygrow to several gigabytes in size. Because users or software developersoften maintain several virtual image files or VHDs, maintaining andefficiently storing the virtual machine image files can be difficult.The problem becomes worse in large organizations where multiple usersare independently maintaining their own image libraries. This results inlarge storage space requirements, even though these images typicallyshare large amounts of common operating system or application code.

What is needed are fast and efficient systems and methods for thetransfer and storage of virtual machine image files.

SUMMARY OF THE INVENTION

This invention allows users of virtual machines to compile and manage arepository of virtual machine image files for use within anorganization. Virtual machine image files are stored in a virtualmachine image store by a virtual machine image server. Users of virtualmachine clients connect to the virtual machine image server and arepresented with a list of the available virtual machine image files inthe virtual machine image store. Users of the virtual machine clientsare able to select the desired virtual machine image, and beginexecuting in the selected virtual machine environment as soon as theselected virtual machine image file begins downloading to the clientcomputer.

Alternatively, if there are existing virtual machine image files storedat the virtual machine client, the virtual machine image server firstdetermines the differences between the virtual machine files at thevirtual machine client and the requested virtual machine image file bycomparing signatures of segments comprising the virtual machine imagefiles with the signatures of segments comprising the requested virtualmachine image file. The virtual machine image server then sends onlythose segments that are different than the segments at the virtualmachine client. The virtual machine client desirably constructs therequested virtual machine image from the stored and received segments.

The invention also addresses the issues related to efficiently storingand retrieving a large number of virtual machine image files. When avirtual machine image file is selected for storage it is divided intoseveral segments. Each segment is compared with segments of previouslystored virtual machine image files already in storage. Onlynon-duplicate segments are stored. In addition, a data structure iscreated and stored for the virtual machine image file identifying thesegments comprising the stored virtual machine image file. When thestored virtual machine image file is later requested the data structureis retrieved and the virtual image file is reassembled from the segmentsas indicated in the data structure.

Furthermore, the invention addresses the issues related to updatingvirtual machine image files at virtual machine clients. When an updateis made to a virtual machine image file at the virtual machine imageserver it is desirable that the update be propagated to virtual machineclients efficiently by reusing the virtual machine image files at thevirtual machine clients. Signatures of the segments comprising thepre-update virtual machine image are compared with signatures of thesegments comprising the post-update virtual machine image. Segments thatare different are desirably sent to the virtual machine clients andsubstituted for the segments stored on the virtual machine clients andthe updated virtual machine image files are constructed on the virtualmachine clients from the stored and received segments.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofpreferred embodiments, is better understood when read in conjunctionwith the appended drawings. For the purpose of illustrating theinvention, there is shown in the drawings exemplary constructions of theinvention; however, the invention is not limited to the specific methodsand instrumentalities disclosed. In the drawings:

FIG. 1 is a block diagram illustrating an exemplary virtual machineimage file server system in accordance with the present invention;

FIG. 2 is a flow diagram illustrating an exemplary method fortransmitting a virtual machine image file in accordance with the presentinvention;

FIG. 3 is a flow diagram illustrating an exemplary method for storing avirtual machine image file in accordance with the present invention;

FIG. 4 is a flow diagram illustrating an exemplary method for retrievinga stored virtual machine image file in accordance with the presentinvention; and

FIG. 5 is a block diagram showing an exemplary computing environment inwhich aspects of the invention may be implemented.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 is a block diagram illustrating an exemplary virtual machineimage file server system in accordance with the present invention. Thesystem comprises a plurality of virtual machine clients 110, 112, and113, a virtual machine image server 121, a virtual machine store 131,and a plurality of virtual machine image files 141, 143, and 145comprised within the virtual machine image store 131. While FIG. 1illustrates three virtual machine clients, it is not meant to limit theinvention to three virtual machine clients. There is no limit to thenumber of virtual machine clients that can be supported. Similarly,there is no limit to the number of virtual machine image servers,virtual machine image stores, or virtual machine image files that can besupported.

One or more virtual machines are executed locally on a client computer,such as clients 110, 112, and 113, for example. Using virtual machines,clients 110, 112, and 113 are desirably able to operate in, and changebetween, a variety of operating systems and application environmentssimply by retrieving and loading one of virtual machine image files 141,143, and 145. Each virtual machine image file 141, 143, and 145desirably corresponds to a different operating system and applicationenvironment. Examples of virtual machine image files 141, 143, and 145may include virtual hard drive files (“VHD”). Virtual machines may beexecuted using virtual machine configuration files (“VMC”). The VMC filedesirably comprises the configuration data for the virtual machine; forexample, what resources should be allocated to the virtual machine, andwhat VHDs may be associated with the virtual machine. The VMC file maynot be necessary to configure the virtual machines; for example, thevirtual machine may desirably be able to operate using a default virtualmachine configuration, and may only require a VHD. The VMC file may bestored as an XML file, however any suitable format known in the art maybe used.

The VHD file desirably comprises the operating system and applicationdata that is executed by the virtual machine on the client devices 110,112, and 113. When the virtual machine, as described in the VMC file,‘boots’ into the operating system contained in the VHD file, the VHDfile appears to the operating system as a physical hard drive with datastored in sectors. In addition, there may be multiple VHD filescomprising a particular virtual machine image file 141, 143, and 145,with each VHD file appearing to the operating system as a separate harddrive.

While the embodiments disclosed herein describe virtual machine imagefiles 141, 143, and 145 as comprising VHD files, it is for illustrativepurposes only, and is not meant to limit the invention to virtualmachine image files comprised only of VHD files. The invention isapplicable to virtual machines configurations using any system, methodor technique known in the art for representing and operating virtualmachines.

The virtual machine image server 121 desirably controls the virtualmachine image files 141, 143, and 145 available for use by the virtualmachine clients 110, 112, and 113. The virtual machine image server 121may be connected to the virtual machine clients by a local area network,or a wide area network, for example the Internet. The virtual machineimage server 121 may operate on a single computer, or may be executedacross multiple distributed computers, for example. The virtual machineimage server 121 desirably communicates with the virtual machine clients110, 112, and 113 using a standard high-level network protocol, forexample HTTP, SMB or FTP. However, any system, method, or techniqueknown in the art for networking may be used.

The virtual machine image server 121 is desirably connected to a virtualmachine image store 131. The virtual machine image store 131 desirablycomprises the virtual machine image files 141, 143, and 145 availablefor use by the clients 110, 112, and 113. As described further withrespect to FIGS. 3 and 4, the virtual machine image files 141, 143, and145 are desirably stored by dividing each file into segments, anddesirably only storing segments that have not been previously stored inthe virtual machine image store 131. However, the virtual machine imagefiles 141, 143, and 145 can be stored using any system, method, ortechnique known in the art for data storage.

The virtual machine image store 131 may operate at a single computer, ornode on a network; however, the virtual machine image store 131 may alsobe distributed across multiple computers or storage devices. The virtualserver 131 may also operate at the same computer, or otherwise part of,the virtual machine image server 121.

The virtual machine clients 110, 112, and 113 desirably send requestsfor virtual machine image files 141, 143, and 145, such as VHDs forexample, to the virtual machine image server 121. The virtual machineimage server 121 desirably logs, or otherwise records requests forvirtual machine image files. The virtual machine clients 110, 112, and113 may automatically detect all the virtual machine image servers 121available on the network, for example.

Each virtual machine image file 141, 143, and 145 may be stored withassociated meta-information. This meta-information may be used byvirtual machine clients 110, 112, and 113 to determine which of thevirtual machine image files 141, 143, and 145 to select for use. Usersof virtual machine clients are desirably able to sort or search theavailable virtual machine image files 141, 143, and 145 using theassociated meta-information.

The requests for virtual machine image files 141, 143, and 145 may begenerated automatically by the virtual machine clients 110, 112, and113, as part of a boot process. For example, in an office environmentwhere frequent updates are made to the operating systems andapplications on computers used by workers, it may be difficult to keepeach worker's system up to date. Accordingly, each of the workercomputers (virtual machine clients 110, 112, and 113) may execute avirtual machine, with the virtual machine configured to retrieve aparticular VHD (one of virtual machine image files 141, 143, and 145)residing on the virtual machine image server 121 at startup. When anupdate to the worker computers are required, such as a operating systempatch for example, the system administrator need only apply the patch tothe VHD on the virtual machine image server 121. The next time theworkers turn on their computers, they will desirably boot into theupdated VHD file.

In an alternative embodiment, each worker desirably boots from a VHDfile stored locally at each of the virtual machine clients 110, 112, and113. The virtual machine image server 121 desirably maintains a list ofeach virtual machine client 110, 112, and 113, and their correspondingVHD file or files. After a user or administrator makes a change to oneof the stored VHD files, each virtual machine client 110, 112, and 113that uses one of the affected VHD files is desirably notified that achange has been made, and an updated VHD should be retrieved and stored.Alternatively, each of the virtual machine clients 110, 112, and 113 mayperiodically poll the virtual machine image server 121 to determine ifthere has been an update to one of their corresponding VHDs. The virtualmachine image server 121 may automatically update the stored VHD fileson the virtual machine clients 110, 112, and 113 without userintervention.

Alternatively, network bandwidth can be preserved by updating the storedVHDs at the virtual machine clients 110, 112, and 113 using remotedifferential compression (“RDC”) as described in pending U.S. patentapplication Ser. Nos. 10/844,893, 10/844,906, 10/844,907, and10/984,980. It is highly likely that the updated VHD and stored VHD onthe virtual machine clients 110, 112, and 113 contain a large amount ofduplicate data. The updated VHD and stored VHD are desirably dividedinto segments. Signatures are desirably computed for each of thesegments. The signatures of the stored VHD segments are desirablycompared using RDC with the signatures of the updated VHD segments.Network bandwidth may be conserved by only transmitting the segments ofthe updated VHD that are different from the segments of the VHD storedin the virtual machine clients 110, 112, and 113, for example. Thepreexisting virtual machine images may be selected by the virtualmachine client using the similarity detection approach as described inU.S. patent application Ser. No. 10/825,735; however, any system, methodor technique known in the art may be used.

Developers may also use the virtual machine image server 121. Forexample, a user or development team may be programming an application.In order to test the application in a variety of user environments andoperating systems, the users or developers may need to quickly switchbetween operating system environments. Accordingly, the user ordevelopment team desirably stores in virtual machine image store 131 aplurality of virtual machine image files, each virtual machine imagefile desirably corresponding to an operating system environment thatthey may desire to test the application in. When the users or developersdesire to load a particular operating system on one oaf the virtualmachine clients 110, 112, and 113, the users desirably connect to thevirtual machine image server 121. The users are then desirably presentedwith a list of the available virtual machine image files at virtualmachine store 131. Alternatively, the users may be presented withmeta-information associated with the virtual machine image file. Theusers can then select one of the stored virtual machine image files, andthe selected virtual machine image file immediately begins to downloadto one of the virtual machine clients 110, 113, and 113. The virtualmachine image file is desirably downloaded using the method as describedfurther with respect to FIG. 2, for example, allowing for the virtualmachine clients 110, 112, and 113 to begin executing in the selectedenvironment before the virtual machine image file has finisheddownloading.

Alternatively, network bandwidth can be preserved by transferring theselected virtual machine image file using RDC. There may be one or morepreexisting virtual machine images stored at one of the virtual machineclients 110, 112, and 113. These preexisting virtual machine images mayshare segments with the selected virtual machine image at virtualmachine store 131. Network bandwidth may be conserved by onlytransmitting the segments of the selected virtual machine image that aredifferent from the segments in the preexisting virtual machine images,for example.

In another embodiment of the present invention, the virtual machineimage store 131, instead of storing virtual machine image filescorresponding to a variety of operating system and applicationenvironments, stores virtual machine configuration file templatescorresponding to a variety of operating system and applicationenvironments. These templates are then desirably used by the virtualmachine image server 121 to generate a virtual machine image filecorresponding to the requested operating system and applicationenvironment requested by the virtual machine clients 110, 112, and 113.

For example, one of virtual machine clients 110, 112, and 113 desirablysends a request to the virtual machine image server 121. The requestdesirably includes a parameter corresponding to the requested operatingsystem, and another parameter corresponding to a requested applicationenvironment. The virtual machine image server 121 desirably retrieves atemplate corresponding to the received parameters and generates avirtual machine image file based on the received parameters. Thegenerated virtual machine image file may be downloaded using RDC or amethod as described with respect to FIG. 2, for example. Once thegenerated virtual machine image file has been downloaded it is desirablydiscarded by the virtual machine image server 121.

FIG. 2 is a flow diagram illustrating an exemplary method fortransmitting a virtual machine image file in accordance with the presentinvention. A request is received for a particular virtual machine imagefile, for example a VHD. The VHD file begins to download to the clientas a background operation at the client device. The virtual machinebegins to “boot” from the partially downloaded VHD file. The virtualmachine attempts to access a particular sector on the VHD. If the sectoris available (i.e., it has been downloaded), the sector is accessed. Ifthe sector is not available (i.e., it has not been downloaded), thebackground application sends a message to the VHD server to fast trackthe needed sector. The VHD server receives the request to fast track andprioritizes the requested sector. After receiving the requested sector,the virtual machine continues accesses the received sector. While FIG. 2is described with respect to VHD files, the method described appliesequally to any other type of virtual machine image file or virtualmachine configurations known in the art.

At 210, a request is desirably received for a VHD. The request may havebeen sent by a client computer device and received by a virtual machineimage server, as described with respect to FIG. 1, for example. A usermay have selected the desired VHD from a list of VHDs available on theVHD server, or the VHD request may have occurred automatically when theclient computer was started, for example. If the requested VHD is notavailable on the VHD server, an error message is desirably generated tothe client device. Otherwise, the VHD file is desirably prepared fordownloading.

At 220, the VHD file desirably begins to download to the requestingclient. As described further with respect to FIGS. 3 and 4, the VHD filemay be stored divided into several pieces or segments. The segmentscomprising the VHD file are desirably located and added to a transmittalqueue for delivery to the requesting client device. The segments aredesirably added to the transmittal queue in the order that they appearin the VHD file.

Alternatively, the VHD file may be downloaded using RDC. Any VHD filesalready present at the requesting client device are first checked todetermine if there are segments that are duplicates of the segmentscomprising the requested VHD. Only non-duplicate segments are desirablyadded to the transmittal queue.

Once in the transmittal queue, the segments desirably begin downloadingto the requesting client. The segments may be further divided intosmaller pieces for transmittal, depending on the capabilities of thenetwork and the underlying transfer protocol used. The VHD file isdesirably downloaded from the virtual machine image server to the clientby a separate background process, such that the virtual machineexecuting on the client computer is desirably not aware that the entireVHD file may not have been downloaded. Any system, method, or techniqueknown in the art for transferring files may be used such as RDC, forexample. The segments in the transmittal queue are desirably downloadedin the order that they were added to the queue.

At 240, the virtual machine desirably attempts to access a sector of theVHD. The virtual machine executing at the client device desirablyattempts to boot from, or otherwise use, the requested VHD that isdownloading to the client device from the virtual machine image server.To an application or operating system executing on the virtual machine,the VHD file appears as a physical hard drive. The bytes comprising theVHD file correspond to sectors on a physical hard drive.

At 250, the client computer desirably determines if the desired sectorhas already been downloaded. Before the sector can be retrieved from theVHD file, the background process, as described above, desirablydetermines if the portion of the VHD that contains the requested sectorhas been downloaded to the client computer. Associated with each VHDfile may be a list of the sectors of the hard drive, and thecorresponding locations of those sectors in the VHD file. Alternatively,there may exist a formula that translates a requested sector number intoa location in the VHD file. Any system, method, or technique known inthe art for determining if a requested portion of a file has been may beused. If it is determined that the requested sector has been downloadedthe embodiment desirably continues at 260. Else, the embodimentcontinues at 270.

At 260, it has been determined that the requested sector has beendownloaded. The requested sector is desirably retrieved and accessed bythe virtual machine. The background process desirably continues todownload the remaining portions of the VHD file from the virtual machineimage server, and any further requests for sectors by the virtualmachine are desirably handled at 240.

At 270, it has been determined that the requested sector has not beendownloaded. The background process desirably sends a message to, orcontacts the virtual machine image server to prioritize the requestedsector in the transmittal queue. As described previously, the VHD fileis downloaded by the background process from the transmittal queuelocated at the virtual machine image server. Because a predeterminedtransmittal order is used for all sectors, such as sequential forexample, and sectors are typically accessed randomly by applications, arequested sector may not be downloaded when needed.

When the request to prioritize the requested sector is received by thevirtual machine image server, the virtual machine image server desirablylocates the segment containing the requested sector in the transmittalqueue, and moves the segment to the front of the queue. After therequested sector has been downloaded to the client, the backgroundprocess desirably allows access to the requested sector. The backgroundprocess desirably continues to download the remaining portions of theVHD file from the virtual machine image server, and any further requestsfor sectors by the virtual machine are desirably handled at 240.

FIG. 3 is a flow diagram illustrating an exemplary method for storingvirtual machine image files, for example VHDs, in accordance with thepresent invention. A VHD is selected to add to storage. The VHD isdivided into segments. A signature, or hash, is computed for eachsegment based on the contents. The computed signatures are storedtogether in a data structure corresponding to the VHD. For each segmentthat is already in storage, the name of the VHD is appended to a list ofVHDs associated with that segment. For each segment that is not instorage, the segment is added to the storage along with a listcontaining the name of the current VHD. While FIG. 3 is described withreference to VHD files only, it is not meant to limit the invention tostoring VHD files. The method described below can be used to store anyother type of virtual machine image file known in the art.

At 310, a VHD is desirably selected for storage. Each VHD may representa hard drive comprising a particular operating system and applicationconfiguration. The VHD desirably allows a user at a client computer toquickly switch between operating system configurations using a virtualmachine. In order to facilitate access to a larger number of VHDs by alarge number of users, the VHDs are desirably stored together. A user orsystem administrator desirably selects the VHD file to store andprovides it to the server using any system, technique, or system knownin the art for transferring data such as through a network, or using aportable storage medium, for example.

At 320, the VHD is desirably divided into segments. The VHD may bedivided into segments according to the method as described in pendingU.S. patent application Ser. Nos. 10/844,893, 10/844,906, and10/844,907, for example. However, any system, method or technique knownin the art for segmenting a large data file can be used. Dividing theVHD into segments allows the server to conserve storage space bydesirably storing any given segment only once. The average sizes of thesegments are desirably chosen by a user or administrator. For example,it may be desirable that segments comprising the first sectors of theVHD be larger on average than segments comprising the end of the VHD.For example, the first sectors of the VHD are more likely to comprisethe operating system data, and are therefore more likely to beduplicates of sectors found in another VHD. In contrast, because thesectors found at the end of the VHD file are more likely to compriseapplication data, it is less likely that a large segment will match anyof the segments already in storage. Any system, technique, or methodknown in the art for determining an optimal segment size can be used.

At 330, a signature is desirably computed for each segment. As describedabove, each segment is desirably compared with stored segments to avoidduplicate storage of segments. In order to avoid comparing segments byteby byte, a signature corresponding to each segment is desirably comparedinstead. The signature is desirably computed using a cryptographicallysecure hash function with a low probability of collision, such as SHA-1for example. However, any system, method, or technique known in the artfor computing a hash function may be used. The resulting signatures aredesirably smaller then their corresponding segment, and thereforerequire significantly less overhead to compare with other signatures.

At 340, a signature vector for the VHD is desirably stored. As describedabove, each segment is desirably only stored if it is not a duplicate ofa segment already found in storage. Instead of storing the entire VHD, avector comprising the signature of each VHD is desirably stored instead.The vector is desirably represented as an array comprising thesignatures for each segment in the order that they appear in theassociated VHD. However, the signature vector can be represented usingany suitable data structure known in the art, such as a linked list forexample.

At 350, the storage is desirably searched for each of the segments. Inorder to determine which segments are not already comprised in storageand may therefore be added to storage, the storage is desirably searchedfor each segment using the computed signatures. Any system, method, ortechnique known in the art for searching for signatures may be used.

If a segment is not found in the storage it is desirably added to thestorage, along with its signature and a list containing the name of theVHD that the segment belongs to at 360.

If a segment is found in the storage, the name or identifier of thecurrent VHD is desirably appended to a list of VHDs that the segment isfound in, and the segment is desirably discarded at 370.

FIG. 4 is a flow diagram illustrating an exemplary method for retrievinga stored virtual machine image file, for example a VHD, in accordancewith the present invention. A request for a VHD is received by the VHDserver from a virtual machine client. An associated signature vector isretrieved for the requested VHD. For each signature listed in thevector, the associated segment is retrieved from storage and added to anoutput queue. Data in the output queue is transmitted to the requestingparty until it is empty. While FIG. 4 is described with reference to VHDfiles only, it is not meant to limit the invention to retrieving storedVHD files only. The method described below can be used to any other typeof virtual machine image file known in the art.

At 410, a request is desirably received for a VHD. The request may bereceived from a virtual machine client. The request may be made using acommon high level network protocol such as HTTP, SMB or FTP, forexample. Any system, method, or technique known in the art for sendingrequests over a network may be used.

At 420, the signature vector corresponding to the requested VHD isdesirably retrieved. As described with respect to FIG. 3, each VHD isdesirably stored as a signature vector, with each signature in thevector corresponding to a stored segment, or portion of the VHD. If asignature vector matching the requested VHD cannot be retrieved, then anerror message is desirably generated. Else, the signature vector isdesirably retrieved from storage.

At 430, for each signature comprised in the signature vector, thecorresponding segment is desirably retrieved and added to an outputqueue. As described further with respect to FIG. 3, the signature vectorcomprises the signature for each of the segments comprising therequested VHD. The signature vector desirably stores the signaturescorresponding to the order that the segments are arranged in the VHD.Accordingly, the signature vector is evaluated sequentially startingwith the first signature in the vector. As each signature in the vectoris evaluated, the corresponding segment is desirably retrieved fromstorage and added to the output queue. The segments may be stored in thesame server as the signature vectors, or the segments may be storedseparately at one or more storage devices.

Alternatively, as described previously with respect to FIG. 1, there maybe additional VHDs stored at the requesting virtual machine client. Eachof the additional VHDs may have segments that have the same signature assegments in the requested VHD. Network bandwidth may be conserved byonly adding segments to the output queue that are not duplicates of thesegments found in VHDs stored at the requesting virtual machine client.This method is described further in pending U.S. patent application Ser.No. 10/948,980.

At 440, the data in the output queue is desirably transmitted until thequeue is empty. The data in the output queue is desirably transmitted tothe originator of the original request for the VHD. The data can betransmitted using any system, technique, or method known in the art fortransmitting data, such as RDC for example. Alternatively, the data inthe output queue may be written to a portable medium, such as a DVD forexample.

Exemplary Computing Environment

FIG. 5 illustrates an example of a suitable computing system environment500 in which the invention may be implemented. The computing systemenvironment 500 is only one example of a suitable computing environmentand is not intended to suggest any limitation as to the scope of use orfunctionality of the invention. Neither should the computing environment500 be interpreted as having any dependency or requirement relating toany one or combination of components illustrated in the exemplaryoperating environment 500.

The invention is operational with numerous other general purpose orspecial purpose computing system environments or configurations.Examples of well known computing systems, environments, and/orconfigurations that may be suitable for use with the invention include,but are not limited to, personal computers, server computers, hand-heldor laptop devices, multiprocessor systems, microprocessor-based systems,set top boxes, programmable consumer electronics, network PCs,minicomputers, mainframe computers, distributed computing environmentsthat include any of the above systems or devices, and the like.

The invention may be described in the general context ofcomputer-executable instructions, such as program modules, beingexecuted by a computer. Generally, program modules include routines,programs, objects, components, data structures, etc. that performparticular tasks or implement particular abstract data types. Theinvention may also be practiced in distributed computing environmentswhere tasks are performed by remote processing devices that are linkedthrough a communications network or other data transmission medium. In adistributed computing environment, program modules and other data may belocated in both local and remote computer storage media including memorystorage devices.

With reference to FIG. 5, an exemplary system for implementing theinvention includes a general purpose computing device in the form of acomputer 5 10. Components of computer 510 may include, but are notlimited to, a processing unit 520, a system memory 530, and a system bus521 that couples various system components including the system memoryto the processing unit 520. The system bus 521 may be any of severaltypes of bus structures including a memory bus or memory controller, aperipheral bus, and a local bus using any of a variety of busarchitectures. By way of example, and not limitation, such architecturesinclude Industry Standard Architecture (ISA) bus, Micro ChannelArchitecture (MCA) bus, Enhanced ISA (EISA) bus, Video ElectronicsStandards Association (VESA) local bus, and Peripheral ComponentInterconnect (PCI) bus (also known as Mezzanine bus).

Computer 510 typically includes a variety of computer readable media.Computer readable media can be any available media that can be accessedby computer 510 and includes both volatile and non-volatile media,removable and non-removable media. By way of example, and notlimitation, computer readable media may comprise computer storage mediaand communication media. Computer storage media includes both volatileand non-volatile, removable and non-removable media implemented in anymethod or technology for storage of information such as computerreadable instructions, data structures, program modules or other data.Computer storage media includes, but is not limited to, RAM, ROM,EEPROM, flash memory or other memory technology, CD-ROM, digitalversatile disks (DVD) or other optical disk storage, magnetic cassettes,magnetic tape, magnetic disk storage or other magnetic storage devices,or any other medium which can be used to store the desired informationand which can accessed by computer 510. Communication media typicallyembodies computer readable instructions, data structures, programmodules or other data in a modulated data signal such as a carrier waveor other transport mechanism and includes any information deliverymedia. The term “modulated data signal” means a signal that has one ormore of its characteristics set or changed in such a manner as to encodeinformation in the signal. By way of example, and not limitation,communication media includes wired media such as a wired network ordirect-wired connection, and wireless media such as acoustic, RF,infrared and other wireless media. Combinations of any of the aboveshould also be included within the scope of computer readable media.

The system memory 530 includes computer storage media in the form ofvolatile and/or non-volatile memory such as ROM 531 and RAM 532. A basicinput/output system 533 (BIOS), containing the basic routines that helpto transfer information between elements within computer 510, such asduring start-up, is typically stored in ROM 531. RAM 532 typicallycontains data and/or program modules that are immediately accessible toand/or presently being operated on by processing unit 520. By way ofexample, and not limitation, FIG. 5 illustrates operating system 534,application programs 535, other program modules 536, and program data537.

The computer 510 may also include other removable/non-removable,volatile/non-volatile computer storage media. By way of example only,FIG. 5 illustrates a hard disk drive 540 that reads from or writes tonon-removable, non-volatile magnetic media, a magnetic disk drive 551that reads from or writes to a removable, non-volatile magnetic disk552, and an optical disk drive 555 that reads from or writes to aremovable, non-volatile optical disk 556, such as a CD-ROM or otheroptical media. Other removable/non-removable, volatile/non-volatilecomputer storage media that can be used in the exemplary operatingenvironment include, but are not limited to, magnetic tape cassettes,flash memory cards, digital versatile disks, digital video tape, solidstate RAM, solid state ROM, and the like. The hard disk drive 541 istypically connected to the system bus 521 through a non-removable memoryinterface such as interface 540, and magnetic disk drive 551 and opticaldisk drive 555 are typically connected to the system bus 521 by aremovable memory interface, such as interface 550.

The drives and their associated computer storage media provide storageof computer readable instructions, data structures, program modules andother data for the computer 510. In FIG. 5, for example, hard disk drive541 is illustrated as storing operating system 544, application programs545, other program modules 546, and program data 547. Note that thesecomponents can either be the same as or different from operating system534, application programs 535, other program modules 536, and programdata 537. Operating system 544, application programs 545, other programmodules 546, and program data 547 are given different numbers here toillustrate that, at a minimum, they are different copies. A user mayenter commands and information into the computer 510 through inputdevices such as a keyboard 562 and pointing device 561, commonlyreferred to as a mouse, trackball or touch pad. Other input devices (notshown) may include a microphone, joystick, game pad, satellite dish,scanner, or the like. These and other input devices are often connectedto the processing unit 520 through a user input interface 560 that iscoupled to the system bus, but may be connected by other interface andbus structures, such as a parallel port, game port or a universal serialbus (USB). A monitor 591 or other type of display device is alsoconnected to the system bus 521 via an interface, such as a videointerface 590. In addition to the monitor, computers may also includeother peripheral output devices such as speakers 597 and printer 596,which may be connected through an output peripheral interface 595.

The computer 510 may operate in a networked environment using logicalconnections to one or more remote computers, such as a remote computer580. The remote computer 580 may be a personal computer, a server, arouter, a network PC, a peer device or other common network node, andtypically includes many or all of the elements described above relativeto the computer 510, although only a memory storage device 581 has beenillustrated in FIG. 5. The logical connections depicted include a LAN571 and a WAN 573, but may also include other networks. Such networkingenvironments are commonplace in offices, enterprise-wide computernetworks, intranets and the internet.

When used in a LAN networking environment, the computer 510 is connectedto the LAN 571 through a network interface or adapter 570. When used ina WAN networking environment, the computer 510 typically includes amodem 572 or other means for establishing communications over the WAN573, such as the internet. The modem 572, which may be internal orexternal, may be connected to the system bus 521 via the user inputinterface 560, or other appropriate mechanism. In a networkedenvironment, program modules depicted relative to the computer 510, orportions thereof, may be stored in the remote memory storage device. Byway of example, and not limitation, FIG. 5 illustrates remoteapplication programs 583 as residing on memory device 581. It will beappreciated that the network connections shown are exemplary and othermeans of establishing a communications link between the computers may beused.

As mentioned above, while exemplary embodiments of the present inventionhave been described in connection with various computing devices, theunderlying concepts may be applied to any computing device or system.

The various techniques described herein may be implemented in connectionwith hardware or software or, where appropriate, with a combination ofboth. Thus, the methods and apparatus of the present invention, orcertain aspects or portions thereof, may take the form of program code(i.e., instructions) embodied in tangible media, such as floppydiskettes, CD-ROMs, hard drives, or any other machine-readable storagemedium, wherein, when the program code is loaded into and executed by amachine, such as a computer, the machine becomes an apparatus forpracticing the invention. In the case of program code execution onprogrammable computers, the computing device will generally include aprocessor, a storage medium readable by the processor (includingvolatile and non-volatile memory and/or storage elements), at least oneinput device, and at least one output device. The program(s) can beimplemented in assembly or machine language, if desired. In any case,the language may be a compiled or interpreted language, and combinedwith hardware implementations.

The methods and apparatus of the present invention may also be practicedvia communications embodied in the form of program code that istransmitted over some transmission medium, such as over electricalwiring or cabling, through fiber optics, or via any other form oftransmission, wherein, when the program code is received and loaded intoand executed by a machine, such as an EPROM, a gate array, aprogrammable logic device (PLD), a client computer, or the like, themachine becomes an apparatus for practicing the invention. Whenimplemented on a general-purpose processor, the program code combineswith the processor to provide a unique apparatus that operates to invokethe functionality of the present invention. Additionally, any storagetechniques used in connection with the present invention may invariablybe a combination of hardware and software.

While the present invention has been described in connection with thepreferred embodiments of the various figures, it is to be understoodthat other similar embodiments may be used or modifications andadditions may be made to the described embodiments for performing thesame function of the present invention without deviating therefrom.Therefore, the present invention should not be limited to any singleembodiment, but rather should be construed in breadth and scope inaccordance with the appended claims.

1. A method for downloading a virtual machine image file to a virtualmachine client, comprising: selecting by the virtual machine client thevirtual machine image file, wherein the selecting comprises selecting afirst parameter identifying an operating system, and a second parameteridentifying an application environment; receiving the selected virtualmachine image file at the virtual machine client from a virtual machineimage file server; and executing the virtual machine image file at thevirtual machine client before the virtual machine image file has beencompletely received by the virtual machine client, wherein the executingcomprises: receiving a request to access a sector of the virtual machineimage file by the virtual machine client; determining if the requestedsector has already been received by the virtual machine client;accessing the requested sector when it is determined that the requestedsector has previously been received by the virtual machine client; andsending a message to the virtual machine image file server to prioritizethe requested sector when it is determined that the requested sector hasnot previously been received by the virtual machine client, and thenaccessing the requested sector when the requested sector is laterreceived by the virtual machine client.
 2. The method of claim 1,wherein retrieving the selected virtual image comprises the virtualmachine image file server generating a virtual machine image file usingthe first and second parameters.
 3. The method of claim 2, wherein thevirtual machine image file is generated using a stored virtual machineimage file configuration template corresponding to the first and secondparameters.
 4. The method of claim 1, wherein the virtual machine imagefile is selected from a plurality of virtual machine image files storedon the virtual machine image file server.
 5. The method of claim 4,wherein selecting a virtual machine image file at the virtual machineclient comprises: receiving meta-information associated with theplurality of virtual machine image files; and selecting the virtualmachine image file using the received meta-information.
 6. The method ofclaim 1, wherein the sectors are received using remote differentialcompression.
 7. A virtual machine client comprising: a processoroperative to execute computer-executable instructions; and memory havingstored therein computer-executable instructions for performing stepscomprising: select a virtual machine image file for transfer from avirtual machine image file server, and execute the virtual machine imagefile before the virtual machine image file has completely transferred tothe virtual machine client from the virtual machine image file server,wherein the executing comprises: receiving a request to access a sectorof the virtual machine image file by the virtual machine client;determining if the requested sector has already been transferred to thevirtual machine client; accessing the requested sector when it isdetermined that the requested sector has previously been transferred tothe virtual machine client; and sending a message to the virtual machineimage file server to prioritize the requested sector when it isdetermined that the requested sector has not previously been transferredto the virtual machine client, and then accessing the requested sectorwhen the requested sector is later transferred to the virtual machineclient.
 8. The virtual machine client of claim 7, wherein the virtualmachine image file is transferred using remote differential compression.9. The virtual machine client of claim 7, wherein the virtual machineclient is adapted to select a first parameter identifying an operatingsystem, and a second parameter identifying an application environment.10. The virtual machine client of claim 7, wherein the virtual machineimage file is selected by the virtual machine client from a plurality ofvirtual machine image files stored on the virtual machine image server.11. The virtual machine client of claim 7, wherein the virtual machineimage file is selected by the virtual machine client at startup.
 12. Thevirtual machine client of claim 7, wherein the virtual machine client isadapted to detect virtual machine image file servers on a network.
 13. Acomputer-readable storage medium with computer-executable instructionsstored thereon for performing the method of: select a virtual machineimage file for transfer from a virtual machine image file server, andexecute the virtual machine image file before the virtual machine imagefile has completely transferred to a virtual machine client from thevirtual machine image file server, wherein the executing comprises:receiving a request to access a sector of the virtual machine image fileby the virtual machine client; determining if the requested sector hasalready been transferred to the virtual machine client; accessing therequested sector when it is determined that the requested sector haspreviously been transferred to the virtual machine client; and sending amessage to the virtual machine image file server to prioritize therequested sector when it is determined that the requested sector has notpreviously been transferred to the virtual machine client, and thenaccessing the requested sector when the requested sector is latertransferred to the virtual machine client.